[Starlink] some post Starship launch thoughts

[Eugene Y Chang]

Engine thrust is a combination of the mass of the gas and the temperature (a measure of velocity).

So coolling the gas (aka absorbing the heat) is part of absorbing the momentum from the engine thrust.

The more speed in the rocket exhaust, the more wear and tear of the equipment in the exhaust’s path.

I suggest it could be calculated
how much speed of the rocket engine exhaust needs to be reduced for a good life of the launch infrastructure
and how much energy needs to be taken out of the rocket exhaust to reduce the exhaust gas to the desired speed.
how much water is needed to absorb the energy needed to reduce the exhaust speed.

Mass and energy are both conserved. They know how much energy is at the rocket engine nozzle (point of max temperature and speed of the gas). The mass and energy have to go somewhere.

To say “only testing will tell us for sure” is to suggest cut-and-try engineering.
Of course, testing is used to confirm the calculations.


Gene
----------------------------------------------
Eugene Chang
IEEE Communications Society & Signal Processing Society,
    Hawaii Chapter Chair
IEEE Hawaii Section, Industry Engagement Coordinator
IEEE Senior Life Member
eugene.chang at ieee.org
m 781-799-0233 (in Honolulu)



> On Apr 25, 2023, at 1:22 PM, David Lang <david at lang.hm> wrote:
> 
> On Tue, 25 Apr 2023, Eugene Chang wrote:
> 
>> I found this YouTube of a deluge system test.
>> It doesn’t look like it uses enough water to succeed.
>> My intuition is the mass of water needed is approximately equal to the rocket’s mass.
> 
> nowhere close. The pad 39a where the Saturn 5 launched has a 300,000 gal take, which is ~2.4M pounds, but the Saturn 5 launch weight was around 6.5M pounds
> 
>> Maybe the system doesn’t have to fully absorb the momentum of the engine exhaust. Still, 70% would be a much greater mass than what the video shows.
> 
> it doesn't, it's not absorbing the momentum of the engine exhaust, it's vaporizing to cool the area and disrupt the airflow so the exhaust is less of a blowtorch when it hits a solid surface, and absorb enough sound to prevent it from damaging the rocket.
> 
> distance helps with both of these, as do the materials that the exhaust finally hits. Regular concrete has too much moisture in it and the water flashes to steam and breaks the concrete (concrete is strong in compression, weak in tension).
> 
> Elon mentioned a few weeks ago that even steel plates would wear down quickly under the exhaust, and that water cooled plates were needed in the long run (and they started building water cooled plates to put under the launch mount)
> 
> will that be enough? only testing will tell us for sure.
> 
> not all rockets use a flame trench, and some have very little deluge
> 
> David Lang

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